#include <iostream>

using namespace std;

typedef long long LL;
const int N = 5e4 + 10;

LL l,n,m;
LL a[N];

LL calc(LL x)
{
    LL cnt = 0;
    for(int i = 0;i <= n;i++)
    {
        int j = i + 1;
        while(j <= n && a[j] - a[i] < x)
        {
            j++;
        }
        cnt += j - i - 1;
        i = j - 1;
    }
    return cnt;
}

int main()
{
    cin >> l >> n >> m;
    for(int i = 1;i <=n;i++)
    {
        cin >> a[i];
    }
    a[n + 1] = l;
    n ++;
    LL left = 1,right = l;
    while(left < right)
    {
        LL mid = (left + right + 1)/2;
        if(calc(mid) <= m)
        {
            left = mid;
        }
        else
        {
            right = mid - 1;
        }
    }
    cout << left << endl;
    return 0;
}